The document "Birefringent Stress Location Sensor" (R. B. Franks et al., SPIE, vol. 586, Fiber Optic Sensors 1985) shows a distributed sensor using a frequency modulated laser source injecting a single mode into a birefringent or monomode optical fiber used as a bimodal fiber. Under the influence of an external parameter, a disturbance, in the occurrence of microbends, generates mode coupling giving rise to the propagation of a second mode in the fiber. Detection is effected by intermodal interferometry. Determination of the position of the disturbance is deduced from measurement of the beat frequency of the detection signal.
There are several problems associated with the above-discussed sensor. Specifically, measurement of the coupling efficiency and determination of the position of the coupling along the fiber are rendered difficult by problems of:
linearity of the frequency gradient of the modulation signal; PA0 stability of the source; PA0 backscattering; PA0 attenuation of the detection owing to the polarization and to the mixing of modes. PA0 the emission of a light wave; PA0 the injection into a bimodal optical fiber of said light wave; and PA0 guidance of said light wave by means of the bimodal optical fiber; PA0 measurement of the modification made to the interferometer; PA0 the detection, in intensity, of the interference product; and PA0 determination of the position, along the bimodal optical fiber, of the disturbance which created the intermodal coupling.